In recent years, there has been a great expansion in the research, development and actual employment of highly sophisticated well logging systems and well logging tools. These systems and tools strain the data carrying capacity of conventional cables. Optical well logging cables have been developed to satisfy the needs for larger data carrying capacity for these more sophisticated well logging systems. In addition, the advancement of geothermal exploration and very deep oil and/or gas wells has forced a need for systems and tools capable of withstanding higher temperatures.
The data from conventional logging systems is in the form of electrical pulses. LED's or lasers are commonly used to convert the electrical pulses into light pulses which are transmitted through the optical fiber. However, these light sources are generally not suitable for use at temperatures greater than about 200.degree. C.
U. S. patent application Ser. No. 285,146, now abandoned filed July 20, 1981 and completely incorporated herein by reference, overcomes this problem by injecting the light into the fiber at the surface and using downhole modulators to modulate either the amplitude or the phase of the light in accordance with electrical signals received from the well logging tool. This system requires four crystals for amplitude modulation. Maintaining the alignment is extremely difficult during the insertion into and extraction from a well bore which penetrates either a geothermal formation or a high-temperature hydrocarbon containing formation. In addition, it is also extremely difficult to obtain four uniform of equivalent thermal coefficients of expansion, doping parameters and purity. Any variation of these parameters will result in the loss of signals or non-uniform signal modulations and variations.
R. K. Swanson et al in "Feasibility Investigation and Design Study of Optical Well Logging Methods for High-Temperature Geothermal Wells" propose the use of an electro-optic waveguide modulator using thin film lithium tantalate, LiTaO.sub.3, crystal. Although feasible, this waveguide configuration is only suitable for single mode optical fiber operation. As the length of the optical fiber over which the signal has to traverse is increased, the coupling efficiencies out of and into the single mode fibers place unrealistic power requirements on the LED's or the lasers necessary to transmit the signal down the optical fiber through the electro-optic modulator and back through the optical fiber to the surface. These thin film modulators increase the power requirements of the system on the order of a 100 times. This requirement exceeds the power budget of any system for practical applications. Furthermore, these thin film single mode crystal modulators are extremely inefficient in that most of the light in the single mode optical fiber is lost because the coupling efficiency into the waveguide is on the order of only about 1%. This is a critical limitation because if the light cannot pass through the crystal, then it cannot be extracted to re-enter a return fiber to be passed back up the well bore.
Thus, it would be desirable to have a high-temperature electro-optic modulator which does not require the careful alignment of four crystals. In addition, it would be desirable to have a high-temperature electro-optic modulator for use down a well bore which does not require more than a single crystal so as to avoid the problems of temperature induced birefringence. It would also be desirable to have an electro-optic modulator which incorporates an electro-optic crystal which increases in performance as the temperature rises and in addition exhibits temperature stability. Furthermore, it would be desirable to have a high-temperature electro-optic modulator which can function with multi-mode optical fibers. Additionally, it would be desirable to have a modulator which can cause the amplitude modulation of a light signal upon passing through only a single electro-optic crystal.